The invention relates to a method for signal evaluation, in particular for measuring the distances between predeterminable signal edges in the evaluation of speed sensor signals.
In the evaluation of speed sensor signals, it is known that there are limits beneath which a reliable evaluation is no longer possible.
For different applications, for example in antilock control systems (ABS), traction control systems (ASR), and vehicle dynamics control (VDC), it is necessary to know the wheel speeds which are determined with the aid of speed sensors. These speed sensors have pulse wheels which have a predeterminable number of teeth, for example 48, and are connected to the wheel or wheels and/or the associated axles, and rotate along with them. The distance from tooth to tooth of such pulse wheels in terms of the wheel circumference is usually approximately 4 cm. When the pulse wheels are scanned, a pick-up, for example an inductive pick-up, generates an output signal which mirrors the surface of the relevant pulse wheel. After a signal processing, a square-wave signal is produced, which is processed in an associated evaluation device. Usually, the evaluation device includes a microcontroller in which each tooth edge of the pulse wheel and/or each signal edge of the signal triggers an interrupt. By measuring the signal intervals, time intervals are produced in the microcontroller, which are in turn proportional to the speed of the wheel.
A method for determining wheel speeds which operates in accordance with the above-described method has been disclosed, for example, by DE-OS 44 444 08 in connection with an antilock system. In the known method, however, no indication is given as to how it is possible to reduce the lower limit value of the wheel speed which can still be evaluated. Usually, wheel speeds that are lower than 2.75 km/h can no longer be evaluated since at that point, the time intervals between the signal edges become so great (116.5 milliseconds) that they can no longer be evaluated using a conventional 16 bit timer, which can only detect a value range from 0 to 65535. These interrelated factors apply to the normally used sensors with pulse wheels that have 48 teeth and a tooth spacing of 4 cm in terms of the wheel circumference.
The method according to the invention has the advantage over the prior art that in a signal evaluation, particularly in the evaluation of square-wave signals with the aid of an n-bit timer, a timer can be used which has fewer bits than were originally required. This advantage is achieved by virtue of the fact that a timer, which is limited to a design-contingent number of n bits, is expanded in its function so that it can operate in a manner which would have originally required a greater number of bits.
For example, in the evaluation of the output signal of a speed or velocity sensor, a 16 bit timer can advantageously be used and despite this fact, a limit speed can be achieved for which an 18 bit timer would originally have been required. Advantageously, the missing two bits are retained by means of increased effort in the calculation process of the software of the microcontroller. In this connection, it is advantageous that only a small amount of RAM storage space is required. Through the use of drum stores, only 4 bits per drum store are advantageously required; a check bit and 3 bits as an overflow counter are required.
The advantage to evaluating wheel sensor signals using the method according to the invention is that the lower limit value of the evaluatable wheel speed can be significantly reduced in comparison to conventional systems so that signal evaluations can advantageously be carried out almost down to the point at which the wheel stops. At least in connection with the determination of wheel speeds, it is possible to.reliably execute the speed measurement down to 0.5 km/h.
This advantage is achieved by virtue of the fact that a method for signal evaluation is executed with which the distance between predeterminable signal edges of the signal to be evaluated is determined by virtue of the fact that with the occurrence of signal edges, timer values are stored and the difference between predeterminable timer values is calculated. The method is executed in a microcontroller which has at least one CPU, one n bit timer, a counting mechanism, and a storage mechanism, in particular at least one drum store. The numerical values obtained when there is a timer overflow are likewise determined and are also taken into account in the determination of the distances between the signal edges.
Advantageously, the method for signal evaluation according to the invention is used in conjunction with the determination of wheel speeds in antilock control systems, traction control systems, and/or vehicle dynamics control systems, as a result of which a reduction of the minimal evaluatable speed is achieved in these systems as well. This applies to both the wheel speeds and the vehicle speed determined based on them. In future vehicles which are equipped with longitudinal control systems, e.g. ACC (adaptive cruise control), it is therefore possible to comfortably brake to a stop.
The methods according to the invention can naturally also be used in other systems; their use is particularly advantageous if signals are to be evaluated in which the interval between a first and second event (e.g. between two signal edges) to be evaluated can be greater and if the capacity (bit number) of the timer used should be or is limited.